Kaleido Biosciences (KLDO) 10-K 2 Mar 20 2019 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Feb. 27, 2020	Jun. 28, 2019
Cover [Abstract]
Entity Registrant Name	KALEIDO BIOSCIENCES, INC.
Document Type	10-K
Amendment Flag	false
Trading Symbol	KLDO
Current Fiscal Year End Date	--12-31
Entity Common Stock, Shares Outstanding		30,420,549
Entity Central Index Key	0001751299
Entity Filer Category	Non-accelerated Filer
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Security Exchange Name	NASDAQ
Entity File Number	001-38822
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-3048279
Entity Address, Address Line One	65 Hayden Avenue
Entity Address, City or Town	Lexington
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02421
City Area Code	617
Local Phone Number	674-9000
Document Transition Report	false
Document Annual Report	true
Title of 12(b) Security	Common Stock, $0.001 Par Value
Entity Ex Transition Period	false
Entity Public Float			$ 119.6
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 71,241	$ 76,086
Prepaid expenses and other current assets	2,038	157
Total current assets	73,279	76,243
Property and equipment, net	6,742	4,693
Restricted cash	2,285	2,180
Deferred issuance costs		2,209
Total assets	82,306	85,325
Current liabilities:
Accounts payable	2,016	3,442
Accrued expenses and other current liabilities	8,361	7,859
Total current liabilities	10,377	11,301
Long term debt, net of unamortized debt discount	20,391	14,831
Restricted shares repurchase liability	3	720
Other liabilities	2,652	278
Warrant liability		1,213
Total liabilities	33,423	28,343
Redeemable convertible preferred stock (Note 8)	0	153,226
Commitments and contingencies (Note 7)
Stockholders’ equity (deficit):
Preferred shares, $0.001 par value, 10,000,000 and no shares authorized; no shares issued or outstanding at December 31, 2019 and 2018, respectively	0	0
Common shares, $0.001 par value, 150,000,000 and 66,000,000 shares authorized; 30,129,096 and 6,115,535 shares issued; 30,127,846 and 5,786,911 shares outstanding at December 31, 2019 and 2018, respectively	30	6
Additional paid-in capital	241,412	9,978
Accumulated deficit	(192,559)	(106,228)
Total stockholders' equity (deficit)	48,883	(96,244)
Total liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)	$ 82,306	$ 85,325

Consolidated Balance Sheets - P

Consolidated Balance Sheets - Parenthetical - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Preferred Stock
Preferred shares par value (in dollars per share)	$ 0.001	$ 0.001
Preferred shares authorized (in shares)	10,000,000	0
Preferred shares issued (in shares)	0	0
Preferred shares outstanding (in shares)	0	0
Common Stock
Common shares par value (in dollars per share)	$ 0.001	$ 0.001
Common shares authorized (in shares)	150,000,000	66,000,000
Common shares issued (in shares)	30,129,096	6,115,535
Common shares outstanding (in shares)	30,127,846	5,786,911

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Operating expenses:
Research and development	$ 64,232	$ 42,062
General and administrative	22,428	18,621
Total operating expenses	86,660	60,683
Loss from operations	(86,660)	(60,683)
Other (expense) income:
Interest income	1,693	1,118
Interest expense	(977)	(1,005)
Change in fair value of warrant liability	252	(918)
Loss on extinguishment of debt	(580)	0
Other expense	(59)	(256)
Total other income (expense), net	329	(1,061)
Net loss	$ (86,331)	$ (61,744)
Net loss per share —basic and diluted	$ (3.36)	$ (12.09)
Weighted-average common shares outstanding used in net loss per share —basic and diluted	25,703,269	5,108,147

Consolidated Statements of Rede

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Redeemable Convertible Preferred Stock
Beginning balance at Dec. 31, 2017	$ (43,679)	$ 5	$ 800	$ (44,484)
Beginning balance (in shares) at Dec. 31, 2017					26,927,398
Beginning balance at Dec. 31, 2017					$ 52,494
Beginning balance (in shares) at Dec. 31, 2017		4,711,963
Conversion of preferred stock warrant to common stock warrant upon closing of initial public offering	0
Issuance of Series C convertible preferred stock (net of issuance costs of $241)					$ 100,732
Issuance of Series C convertible preferred stock (net of issuance costs of $241) (in shares)					10,107,404
Exercise of stock options	120		120
Exercise of stock options (in shares)		116,156
Stock-based compensation	6,964		6,964
Vesting of restricted shares	$ 2,095	$ 1	2,094
Vesting of restricted shares (in shares)		958,792
Conversion of redeemable convertible preferred stock into common stock			0
Conversion of redeemable convertible preferred stock into common stock (in shares)	18,517,386
Net loss	$ (61,744)			(61,744)
Ending balance at Dec. 31, 2018	$ (96,244)	$ 6	9,978	(106,228)
Ending balance (in shares) at Dec. 31, 2018	37,034,802				37,034,802
Ending balance at Dec. 31, 2018	$ 153,226				$ 153,226
Ending balance (in shares) at Dec. 31, 2018		5,786,911
Conversion of preferred stock warrant to common stock warrant upon closing of initial public offering	871		871
Issuance of common stock, net of issuance costs of $8,411	65,946	$ 5	65,941
Issuance of common stock, net of issuance costs of $8,411 (in shares)		5,000,000
Exercise of common stock warrant (in shares)		51,015
Exercise of stock options	$ 630		630
Exercise of stock options (in shares)	445,160	445,160
Stock-based compensation	$ 10,068		10,068
Vesting of restricted shares	717		717
Vesting of restricted shares (in shares)		327,374
Conversion of redeemable convertible preferred stock into common stock	153,226	$ 19	153,207
Conversion of redeemable convertible preferred stock into common stock (in shares)					(37,034,802)
Conversion of redeemable convertible preferred stock into common stock					$ (153,226)
Conversion of redeemable convertible preferred stock into common stock (in shares)		18,517,386
Net loss	(86,331)			(86,331)
Ending balance at Dec. 31, 2019	48,883	$ 30	$ 241,412	$ (192,559)
Ending balance at Dec. 31, 2019	$ 0
Ending balance (in shares) at Dec. 31, 2019		30,127,846

Consolidated Statements of Re_2

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' (Deficit) Equity (Unaudited) - Parenthetical - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Statement Of Stockholders Equity [Abstract]
Common stock, issuance costs	$ 9,055
Issuance costs of Series C convertible preferred stock		$ 241

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Operating activities:
Net loss	$ (86,331)	$ (61,744)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	1,318	792
Loss on extinguishment of debt	580	0
Stock-based compensation	10,068	6,964
Amortization of debt discount	37	0
Non-cash interest expense	0	62
Loss on disposal of fixed asset	0	1
Change in fair value of warrant liability	(342)	918
Change in fair value of derivative	90	195
Changes in operating assets and liabilities:
Due to/from related party	0	(102)
Prepaid expenses and other assets	(1,881)	40
Accounts payable	(684)	1,837
Accrued expense and other liabilities	1,349	4,721
Net cash used in operating activities	(75,796)	(46,316)
Investing activities:
Purchase of property and equipment	(3,586)	(3,002)
Net cash and restricted cash used in investing activities	(3,586)	(3,002)
Financing activities:
Proceeds from issuance of debt	37,500	0
Repayments on debt	(30,000)	0
Payments for deferred issuance costs related to IPO	0	(1,815)
Payments of issuance and extinguishment costs related to debt	(858)	(25)
Proceeds from preferred stock financing, net of issuance costs	0	100,732
Proceeds from exercise of stock options	630	120
Payments related to capital lease	(91)	(105)
Issuance of common stock, net of issuance costs	67,761	0
Settlement of derivative liability	(300)	0
Net cash provided by financing activities	74,642	98,907
Net (decrease) increase in cash, cash equivalents, and restricted cash	(4,740)	49,589
Cash, cash equivalents, and restricted cash, beginning of period	78,266	28,677
Cash, cash equivalents, and restricted cash, end of period	73,526	78,266
Supplemental cash flow information
Interest paid	881	943
Supplemental disclosure of non-cash investing and financing activities
Vesting of restricted stock	717	2,095
Reclassification of warrants to additional paid-in capital	871	0
Derivative liability related to debt	0	15
Deferred issuance costs in accounts payable and accrued expenses	0	394
Conversion of preferred stock to common stock upon closing of the initial public offering	153,226	0
Purchase of property and equipment in accounts payable and accrued expenses	$ 385	$ 604

Nature of the Business, Basis o

Nature of the Business, Basis of Presentation, and Going Concern	12 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business, Basis of Presentation, and Going Concern	1. Nature of the Business, Basis of Presentation, and Going Concern Kaleido Biosciences, Inc. and its wholly owned subsidiaries (the “Company”) is a clinical-stage healthcare company that was incorporated in Delaware on January 27, 2015 and has a principal place of business in Lexington, Massachusetts. The Company was formed to use its differentiated, chemistry-driven approach to leverage the potential of the microbiome organ to treat disease and improve human health. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, successful development of technology, obtaining additional funding, protection of proprietary technology, compliance with government regulations, risks of failure of preclinical studies (including ex vivo On March 4, 2019, the Company completed its initial public offering (the "IPO"), pursuant to which it issued and sold 5,000,000 shares of common stock. The aggregate net proceeds received by the Company from the IPO were $69.8 million, after deducting underwriting discounts and commissions, but before deducting offering costs payable by the Company, which totaled $3.8 million. Upon the closing of the IPO, all outstanding shares of convertible preferred stock converted into 18,517,386 shares of common stock. Going Concern During the years ended December 31, 2019 and 2018, the Company incurred net losses of $86.3 million and $61.7 million, respectively, and reported cash used in operations totaling $75.8 million and $46.3 million, respectively. In addition, as of December 31, 2019, the Company had an accumulated deficit of $192.6 million. The Company expects to continue to generate operating losses and use cash in operations in the foreseeable future. As of December 31, 2019, the Company had cash and cash equivalents of $71.2 million, and management expects that the cash and cash equivalents at December 31, 2019 will be sufficient to fund its operating expenses and capital expenditure requirements into the first quarter of 2021. Based on its recurring losses from operations incurred since inception, expectation of continuing operating losses for the foreseeable future, and need to raise additional capital to finance its future operations, the Company has concluded that there is substantial doubt regarding the Company’s ability to continue as a going concern within one year after the data that these consolidated financial statements are issued. The Company will require substantial additional capital to fund its research and development and ongoing operating expenses. These capital requirements are expected to be funded through debt and equity offerings as well as possible strategic collaborations with other companies. If the Company is unable to raise additional funds when needed, it may be required to delay, reduce or eliminate our product development or future commercialization efforts, or grant rights to develop and market product candidates that the Company would otherwise prefer to develop and market itself. While there can be no assurance the Company will be able to successfully reduce operating expenses or raise additional capital, management believes the historical success in managing cash flows and obtaining capital will continue in the foreseeable future. The accompanying consolidated financial statements have been prepared on a going concern basis, which contemplates the realization of assets and the satisfaction of liabilities in the normal course of business. These financial statements do not include any adjustments that might result from the outcome of this uncertainty.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). Principles of Consolidation The accompanying consolidated financial statements reflect the operations of the Company and its wholly owned subsidiaries. All intercompany accounts and transactions have been eliminated. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Actual results may differ from those estimates. Cash and Cash Equivalents Cash includes cash in readily available checking accounts. The Company’s cash deposits on hand at one financial institution often exceed federally insured limits. Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Restricted Cash Restricted cash is cash that is restricted as to withdrawal or use under the terms of certain contractual agreements. The restricted cash consists of cash collateral for secured letters of credit for the security deposit on the Company’s leased laboratory and office facilities. Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. The Company’s cash equivalents as of December 31, 2019 consisted only of money market funds. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company relies, and expects to continue to rely, on a small number of vendors to manufacture supplies and raw materials for its development programs. These programs could be adversely affected by a significant interruption in these manufacturing services or the availability of raw materials. Property and Equipment Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the respective assets. Laboratory and office equipment, computer equipment and furniture and fixtures are depreciated over a period of five years, and leasehold improvements are amortized over the lesser of the asset’s estimated useful life or the remaining lease term. Major additions and betterments are capitalized; maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operating expenses as incurred. Impairment of Long-Lived Assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset may not be recoverable. When such events occur, the Company compares the carrying amounts of the assets to their undiscounted expected future cash flows. If this comparison indicates that there is an impairment, the amount of the impairment is calculated as the difference between the carrying value and the fair value. The Company has not recorded any impairment charges in the periods presented. Deferred Issuance Costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred issuance costs until such financings are consummated. After consummation of an equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the offering. The Company capitalizes certain legal and other third‑party fees that are directly associated with obtaining access to capital under credit facilities. Deferred financing costs related to a recognized debt liability are recorded as a reduction of the carrying amount of the debt liability and amortized to interest expense using the effective interest method over the repayment term. Segment Information Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision-making group, in making decisions on how to allocate resources and assess performance. The Company’s chief operating decision maker is the chief executive officer (“CEO”). The Company and CEO view the Company’s operations and manage its business as one operating segment. Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and bonuses, stock‑based compensation, employee benefits, facilities costs, laboratory supplies, depreciation, manufacturing expenses and external costs of vendors engaged to conduct preclinical development activities and clinical trials, as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology are expensed as research and development in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed. Research and Manufacturing Contract Costs and Accruals The Company has entered into various research and development and manufacturing contracts. These agreements are generally cancelable, and related payments are recorded as the corresponding expenses are incurred. The Company records accruals for estimated ongoing costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the research studies or clinical trials and manufacturing activities, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be required in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs. Patent Costs All patent‑related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. Income Taxes Deferred tax assets and liabilities are recognized for the expected future tax consequences of events that have been included in the consolidated financial statements or tax returns. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax asset to an amount, which, more likely than not, will be realized. The Company recognizes the tax benefit from any uncertain tax positions only if it is more likely than not that the tax position will be sustained on examination by the taxing authorities, based on the technical merits of the position. The tax benefits recognized in the consolidated financial statements from such a position are measured based on the largest benefit that has a greater than fifty percent likelihood of being realized upon ultimate settlement. Fair Value Measurements Certain assets and liabilities were carried at fair value. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1 – Unadjusted quoted prices in active markets that are accessible to the reporting entity at the measurement date for identical assets and liabilities. • Level 2 – Inputs other than quoted prices in active markets for identical assets and liabilities that are observable either directly or indirectly for substantially the full term of the asset or liability. Level 2 inputs include the following: • quoted prices for similar assets and liabilities in active markets • quoted prices for identical or similar assets or liabilities in markets that are not active • observable inputs other than quoted prices that are used in the valuation of the asset or liabilities (e.g., interest rate and yield curve quotes at commonly quoted intervals) • inputs that are derived principally from or corroborated by observable market data by correlation or other means • Level 3 – Unobservable inputs for the assets or liability (i.e., supported by little or no market activity). Level 3 inputs include management’s own assumptions about the assumptions that market participants would use in pricing the asset or liability (including assumptions about risk). The carrying amount of the Company’s other financial assets and liabilities including cash, accounts payable and long-term debt approximate fair value because of the relatively short period of time between origination and expected realization or settlement. Net Loss Per Share Prior to the closing of its IPO, the Company followed the two-class method when computing net income (loss) per share as the Company had issued preferred stock shares that met the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon the respective rights to receive dividends as if all income for the period had been distributed. No losses were allocated to the preferred stock. Subsequent to the closing of its IPO, b asic net income (loss) per share is computed by dividing the net income (loss) by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) is computed by adjusting net income (loss) to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share is computed by dividing the diluted net income (loss) by the weighted average number of shares of common stock outstanding for the period, including potential dilutive common shares assuming the dilutive effect of common stock equivalents. The following table presents securities that have been excluded from the computations of diluted weighted-average shares outstanding as they would be anti-dilutive due to the net losses: As of December 31, 2019 2018 Options to purchase common stock 7,285,581 6,686,267 Unvested restricted common stock 1,250 328,624 Convertible redeemable preferred stock (as converted to common stock) — 18,517,386 Warrant to purchase redeemable convertible preferred stock (as converted to common stock) — 68,514 7,286,831 25,600,791 Redeemable Convertible Preferred Stock The Company recorded redeemable convertible preferred stock at fair value upon issuance, net of any issuance costs. The Company’s redeemable convertible preferred stock was subject to a dividend when and if declared by the Company’s board of directors (the “Board”). Since inception, no dividend has been declared. The Company classifies stock that is redeemable in circumstances outside of the Company’s control outside of permanent equity. No accretion was recognized as the contingent events that could have given rise to redemption were not deemed probable. Upon completion of the IPO, all the Redeemable Convertible Preferred Stock was converted to shares of common stock. Stock-Based Compensation For stock-based awards, the Company measures the estimated fair value of the stock-based award on the date of grant and recognizes compensation expense for those awards over the requisite service period, which is generally the vesting period of the respective award. For stock-based awards with service-based vesting conditions, the Company records the expense for these awards using the straight-line method. For stock options with performance-based vesting conditions, the Company records the expense for these awards over the requisite service period using an accelerated attribution method to the extent the achievement of the performance condition is probable. The Company accounts for forfeitures as they occur. The Company classifies stock-based compensation expense in its consolidated statements of operations in the same manner in which the award recipient’s cash compensation costs are classified. Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. The Company’s comprehensive net loss equals the reported net loss for all periods presented. Subsequent events The Company evaluates events and/or transactions occurring after the balance sheet date and before the issue date of the financial statements to determine if any of those events and/or transactions require adjustment to or disclosure in the financial statements. Accounting Pronouncements Issued and Not Adopted In February 2016, the FASB issued ASU No. 2016-02, Leases

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The following tables set forth by level, within the fair value hierarchy, the assets and liabilities carried at fair value on a recurring basis (in thousands): Fair Value Measurement as of December 31, 2019 Level 1 Level 2 Level 3 Total Assets: Money market funds included within cash and cash equivalents $ 25,304 — — $ 25,304 Total $ 25,304 — — $ 25,304 Fair Value Measurement as of December 31, 2018 Level 1 Level 2 Level 3 Total Assets: Money market funds included within cash and cash equivalents $ 74,145 — — $ 74,145 Total $ 74,145 — — $ 74,145 Liabilities: Warrant liability $ — — 1,213 $ 1,213 Derivative liability — — 210 $ 210 $ — — 1,423 $ 1,423 The fair value of money market funds was measured by the Company based on quoted market prices. The convertible preferred stock warrant liability consisted of the fair value of warrants to purchase Series A and Series B convertible preferred stock and was based on significant inputs not observable in the market. The Company’s valuation of the convertible preferred stock warrants utilized the Black-Scholes option-pricing model, which incorporated assumptions and estimates to value the convertible preferred stock warrants. The Company assessed these assumptions and estimates on a quarterly basis as additional information impacting the assumptions was obtained. Changes in the fair value of the convertible preferred stock warrants were recognized as other income (expense) in the consolidated statements of operations. The quantitative elements associated with the Company’s Level 3 inputs that impacted the fair value measurement of the convertible preferred stock warrant liability included the fair value per share of the underlying Series A and Series B convertible preferred stock, the remaining contractual term of the warrants, risk-free interest rate, expected dividend yield and expected volatility of the price of the underlying convertible preferred stock. The most significant assumption in the Black-Scholes option-pricing model that impacts the fair value of the convertible preferred stock warrants was the fair value of the Company’s convertible preferred stock as of each re-measurement date. The Company determined the fair value per share of the underlying convertible preferred stock by taking into consideration its most recent sales of its convertible preferred stock as well as additional factors that the Company deemed relevant. The Company historically had been a private company and lacked company-specific historical and implied volatility information of its stock. Therefore, it estimated its expected stock volatility based on the historical volatility of publicly-traded peer companies for a term equal to the remaining contractual term of the warrants. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve for time periods approximately equal to the remaining contractual term of the warrants. The Company has estimated a 0% dividend yield based on the expected dividend yield and the fact that the Company has never paid or declared dividends. Upon the closing of the IPO, the warrants for the purchase of convertible preferred stock automatically became warrants for the purchase of common stock and the Company reclassified the carrying value of the warrants from a liability to additional paid-in capital in the consolidated balance sheet. The warrants were subsequently exercised and net settled. The fair value of the derivative liability recognized in connection with the contingent success fee associated with the amended term loan agreement was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of the derivative liability was determined using the probability-weighted expected return method (“PWERM”), which considered as inputs the probability of occurrence of an event (as defined), the expected timing of a liquidity event, the amount of the success fee and a risk-adjusted discount rate. As of December 31, 2018, the assumed probability of occurrence of the event that was most probable of triggering the payment was 70%, The following table presents a roll-forward of the aggregate fair values of the Company’s liabilities for which fair value is determined by Level 3 inputs (in thousands): Warrant Liability Derivative Liability Balance – January 1, 2018 $ 295 $ — Initial fair value of derivative liability — 15 Change in fair value 918 195 Balance – December 31, 2018 1,213 210 Change in fair value through the exercise /settlement date (342 ) 90 Reclassification to additional paid-in capital in connection with IPO (871 ) — Settlement of liability in connection with IPO — (300 ) Balance – December 31, 2019 $ — $ — There were no transfers among Level 1, Level 2, or Level 3 categories in the periods presented. Financial Instruments Not Recorded at Fair Value The carrying value of cash, cash equivalents, restricted cash, accounts payable and accrued expenses that are reported on the consolidated balance sheets approximate their fair value due to the short-term nature of these assets and liabilities. The carrying value of the long-term debt approximates fair value as evidenced by the recent refinancings.

Property and Equipment, net

Property and Equipment, net	12 Months Ended
	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Property and Equipment, net	4. Property and Equipment, net Property and equipment consist of the following (in thousands): As of December 31, 2019 2018 Laboratory equipment $ 4,526 $ 3,226 Office and computer equipment 1,418 1,337 Leasehold improvements 687 653 Construction in process 2,650 698 Property and equipment – at cost 9,281 5,914 Less accumulated depreciation and amortization (2,539 ) (1,221 ) Property and equipment – net $ 6,742 $ 4,693 Depreciation and amortization expense was $1.3 million and $0.8 million for the years ended December 31, 2019 and 2018, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2019
Payables And Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	5. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following (in thousands): As of December 31, 2019 2018 Payroll and benefits $ 2,426 $ 3,297 Consulting service 230 243 Legal service 171 90 Research and development 4,259 3,718 Capital lease payable – short term 68 91 Other 1,207 420 $ 8,361 $ 7,859

Debt Financing

Debt Financing	12 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Debt Financing	6. Debt Financing 2015 Credit Facility The Company was party to a loan and security agreement, as amended (the “2015 Credit Facility”), under which the Company had borrowed an aggregate of $15.0 million. Borrowings under the 2015 Credit Facility bore interest at an annual rate equal to the lender’s prime rate plus 1.00%, subject to a floor of 5.75%. In October 2019, the Company repaid all borrowings under the 2015 Credit Facility. The aggregate principal amount of the loan outstanding at the time of repayment was $15.0 million. As a result of the repayment the Company paid a prepayment fee of $0.1 million. The Company recognized a loss on the extinguishment of the time of repayment totaling $0.2 million. 2019 Credit Facility In October 2019, the Company entered into a loan and security agreement (the “2019 Credit Facility”) pursuant to which the lender made term loans in an aggregate principal amount of $15.0 million which were used to extinguish the 2015 Credit Facility. Issuance costs totaled $0.1 million. In December 2019, the Company repaid all borrowings under the 2019 Credit Facility. The aggregate principal amount of the loan outstanding at the time of repayment was $15.0 million. The Company also paid the prepayment fee of 2% totaling $0.3 million. The Company recognized a loss on the extinguishment of the 2019 Credit Facility of $0.4 million related to the unamortized debt discount and prepayment fee at the time of repayment. 2019 Credit Agreement On December 31, 2019, the Company entered into a Credit Agreement (the “Credit Agreement”). Under the Credit Agreement, the Company borrowed $22.5 million, and the Company has the option to draw down an additional $12.5 million if certain milestones and conditions are met. The Company incurred fees of $0.3 million, which was paid to the lender on the closing date. These amounts were recorded as a debt discount and are being amortized as interest expense using the effective interest method over the life of the Credit Agreement. The Credit Agreement also includes an end of term charge equal to 7.55% of the aggregate principal amount of all advances. The end of term charge, totaling $1.7 million at December 31, 2019, was recognized as a debt discount and is reflected as a reduction in the carrying value of the debt and recorded in other long-term liabilities. The debt discount created by the end of term charge is being accreted and will be recognized as additional interest expense over the term of the Credit Agreement using the effective interest method. The Credit Agreement contains customary representations and warranties, events of default and affirmative and negative covenants, including, among others, covenants that limit or restrict the Company’s ability to, among other things, incur additional indebtedness, merge or consolidate, make acquisitions, pay dividends or other distributions or repurchase equity, make investments, dispose of assets and enter into certain transactions with affiliates, in each case subject to certain exceptions. As security for its obligations under the Credit Agreement, the Company granted the Lender a first priority security interest on substantially all of the Company’s assets (other than intellectual property), and subject to certain exceptions. The outstanding principal under the Credit Agreement has a 48-month term with interest only payments for the first 15 months, which period can be extended to up to 24 months, depending on the achievement of certain performance milestones. The principal bears an interest rate of equal to the greater of (i) 8.95% plus the prime rate minus 4.75% and (ii) 8.95%. The Credit Agreement includes mandatory prepayment provisions that require prepayment upon the occurrence of a change in control event. Future principal payments under the Credit Agreement as of December 31, 2019 are as follows (in thousands): 2021 5,454 2022 8,182 2023 8,182 2024 682 Total future principal payments 22,500 Less unamortized debt discount 2,109 Total balance $ 20,391

Commitments and contingencies

Commitments and contingencies	12 Months Ended
	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and contingencies	7. Commitments and contingencies Facilities Leases Lexington, MA Lease In March 2018, the Company entered into a non-cancelable ten-year Rent expense for the years ended December 31, 2019 and 2018 was $4.5 million and $1.8 million, respectively. Future minimum lease payments under the non-cancelable operating leases consisted of the following as of December 31, 2019 (in thousands): Year Ending December 31, 2020 5,843 2021 6,026 2022 6,207 2023 6,393 2024 6,584 Thereafter 32,284 $ 63,337

Stockholders_ Equity

Stockholders’ Equity	12 Months Ended
	Dec. 31, 2019
Stockholders Equity Note [Abstract]
Stockholders’ Equity	8. Stockholders’ Equity As of December 31, 2018, convertible preferred stock consisted of the following: As of December 31, 2018 Preferred Stock Authorized Preferred Stock Issued and Outstanding Carrying Value Liquidation Preference Common Shares Issuable Upon Conversion Series A Preferred Stock 14,469,180 14,383,563 $ 10,487 $ 10,500 7,191,781 Series A-1 Preferred Stock 3,057,972 3,057,972 5,168 5,168 1,528,985 Series B Preferred Stock 9,537,276 9,485,863 36,839 36,900 4,742,924 Series C Preferred Stock 10,107,404 10,107,404 100,732 100,973 5,053,696 37,171,832 37,034,802 $ 153,226 $ 153,541 18,517,386 Upon completion of the IPO, all the outstanding shares of the Preferred Stock were converted into an aggregate of 18,517,386 shares of common stock. Common stock Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are not entitled to receive dividends, unless declared by the board of directors. In March 2019, the Company filed an amended and restated certificate of incorporation in the State of Delaware, which, among other things, restated the number of shares of all classes of stock that the Company has authority to issue to 160,000,000 shares, consisting of (i) 150,000,000 shares of common stock, $0.001 par value per share, and (ii) 10,000,000 shares of preferred stock, $0.001 par value per share. The shares of preferred stock are currently undesignated. Stock-based compensation 2015 Stock Incentive Plan The Company’s 2015 Stock Incentive Plan (the “2015 Plan”) provided for the Company to sell or issue incentive stock options or nonqualified stock options, restricted stock, and other equity awards to employees, directors and consultants of the Company. The 2019 Stock Option and Incentive Plan (the “2019 Plan”) became effective in February 27, 2019. Upon effectiveness of the 2019 Plan, the remaining shares available under the 2015 Plan ceased to be available for issuance and no future issuances will be made under the 2015 Plan. The 2019 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock units, restricted stock awards, unrestricted stock awards, cash-based awards and dividend equivalent rights to the Company’s officers, employees, directors and consultants. The number of shares initially reserved for issuance under the 2019 Plan is 2,168,976, has increased on January 1, 2020 and will continue to increase each January 1 thereafter by 4% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or such lesser number of shares determined by the Company’s board of directors or compensation committee of the board of directors. 2019 Employee Stock Purchase Plan The 2019 Employee Stock Purchase Plan (the “2019 ESPP”) became effective on February 27, 2019. A total of 180,748 shares of common stock were reserved for issuance under this plan. In addition, the number of shares of common stock that may be issued under the ESPP automatically increased on January 1, 2020, and will continue to increase each January 1 thereafter, by the lesser of (i) 542,244 shares of common stock, (ii) 1% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or (iii) such lesser number of shares as determined by the administrator of the 2019 ESPP. No shares were issued under the 2019 ESPP in 2019. Stock Option Valuation The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. For options with service-based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. The Company typically grants stock options at exercise prices deemed by the Board to be equal to the fair value of the common stock at the time of grant. In the periods prior to the IPO, the fair value of the common stock has been determined by the Board at each measurement date based on a variety of different factors, including the results obtained from independent third-party appraisals, the Company’s financial position and historical financial performance, the status of development of the Company’s programs, the current climate in the marketplace, the illiquid nature of the common stock, the effect of the rights and preferences of the preferred stockholders, and the prospects of a liquidity event, among others. In the periods following the IPO, the fair value is determined based upon the quoted price of the Company’s common stock. The assumptions that the Company used to determine the grant-date fair value of options granted were as follows: Years Ended December 31, 2019 2018 Expected volatility 66.1% - 84% 46% - 55% Risk-free interest rate 1.42% - 2.54% 2.66% - 3.07% Expected term (in years) 5.50-6.25 5.81-6.25 Expected dividend yield — % — % Stock Options Activity A summary of the Company’s stock option activity and related information is as follows: Options Weighted Average Exercise Price Weighted Average Remaining Life (in Years) Aggregate Intrinsic Value Outstanding as of January 1, 2019 6,686,267 $ 7.50 9.2 68,167 Granted 2,543,902 8.7 Exercised (445,160 ) 1.41 Canceled (1,499,428 ) 8.15 Outstanding as of December 31, 2019 7,285,581 $ 8.15 8.7 5,075 Options exercisable as of December 31, 2019 1,900,395 7.3 8.1 2,158 Options vested or expected to vest as of December 31, 2019 7,285,231 8.2 8.7 5,075 The weighted-average grant date fair value of the options granted during the year ended December 31, 2019 and 2018 was $5.96 and $5.47 per share, respectively. As of December 31, 2019 there was $28.2 million of unrecognized compensation expense, which the Company expects to recognize over the weighted-average remaining term of 2.82 years. Restricted Common Stock During the year ended December 31, 2017, the Company signed agreements with seven employees to early exercise stock options covering 1,295,699 shares to convert such options to restricted common stock prior to the vesting of the underlying shares of common stock. The vesting conditions did not change. The consideration received due to the early exercises from the seven employees was recorded as a restricted share repurchase liability. As of December 31, 2019 and 2018, the outstanding balance of the restricted share repurchase liability was $0.0 million and $0.7 million, respectively. The following table summarizes the Company’s restricted common stock activity for the year ended December 31, 2019: Number of Restricted Shares Weighted-Average Grant Date Fair Value Issued and unvested as of January 1, 2019 328,624 $ 2.19 Vested (327,374 ) Issued and unvested as of December 31, 2019 1,250 $ 2.22 Stock- Based Compensation Expense The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations: Year Ended December 31, 2019 2018 Research and development $ 3,245 $ 1,309 General and administrative 6,823 5,655 $ 10,068 $ 6,964

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes There is no provision for income taxes because the Company has historically incurred net operating losses and maintains a full valuation allowance against its deferred tax assets. The reported amount of income tax benefit for the years ended December 31, 2019 and 2018 differs from the amount that would result from applying domestic federal statutory rates to pretax losses primarily because of changes in the valuation allowance, state taxes, and the generation of research and development credits. Significant components of the Company’s net deferred tax assets at December 31, 2019 and 2018 are as follows: Years ended December 31, 2019 2018 Deferred tax assets Stock-based compensation $ 3,754 $ 1,067 Net operating loss carryforwards 45,374 24,638 Credit carryforwards 6,519 4,252 Intangible assets 180 197 Charitable contributions 1 1 Accrued expenses 1,125 788 Total deferred tax assets 56,953 30,943 Valuation allowance (56,817 ) (30,837 ) Total net deferred tax assets 136 106 Deferred tax liabilities: Fixed assets (136 ) (106 ) Total net deferred tax liability (136 ) (106 ) Total deferred tax assets (liability) $ — $ — A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Years ended December 31, 2019 2018 Federal income tax expense at statutory rate 21.0 % 21.0 % Stock compensation expense (0.5 ) (1.0 ) Fair value change in warrant liability 0.1 (0.4 ) Permanent differences (0.1 ) (0.1 ) Federal research and development credit 1.7 3.0 State research and development credit 0.9 1.1 State income tax, net of federal benefit 6.3 5.8 Other 0.7 (0.3 ) Change in valuation allowance (30.1 ) (29.1 ) Effective income tax rate 0 % 0 % As of December 31, 2019, the Company had net operating loss (NOL) carryforwards for U.S. federal and state tax purposes of $166.8 million and $163.6 million, respectively. Federal NOLs of $38.8 million, generated before 2018, will begin expiring in varying amounts in 2035 unless utilized and the remaining NOL of $128 million, generated after 2018 will be carried forward indefinitely and could be used up to 80% of taxable income of each future tax year. The Commonwealth of Massachusetts does not follow federal on NOL carryforwards and as such the Company’s Massachusetts NOLs of $163.6 million will expire in at various times starting in 2035. As of December 31, 2019, the Company also has federal research and development tax credit carryforwards of approximately $4.5 million, and state research and development tax credit carryforwards of approximately $2.5 million, which may be available to reduce future tax liabilities, and which expire at various dates through 2039. Management of the Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, which are comprised principally of net operating loss carryforwards research and development tax credit carryforwards and capitalized expenditures. Under the applicable accounting standards, management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and concluded that it is more likely than not that the Company will not recognize the benefits of its federal and state deferred tax assets. Accordingly, a full valuation allowance has been established, and the valuation allowance increased $26.0 million and $18.0 million in the years ended December 31, 2019 and 2018, respectively. Utilization of the net operating loss and research and development credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Code due to ownership change limitations that have occurred previously or that could occur in the future. These ownership changes may limit the amount of net operating loss and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. The Company has not completed a study to assess whether a change of ownership has occurred, or whether there have been multiple ownership changes since its formation, due to the significant cost and complexity associated with a study. There could also be additional ownership changes in the future which may result in additional limitations on the utilization of net operating loss carryforwards and tax credits. The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. The Company’s tax returns are open under statute from 2016 to the present. The Company’s policy is to record estimated interest and penalties related to uncertain tax positions in income tax expense. The Company has no amounts recorded for any unrecognized tax positions, accrued interest or penalties as of December 31, 2019 and 2018.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2019
Related Party Transactions [Abstract]
Related Party Transactions	10. Related Party Transactions The Company receives professional services from its principal investor, Flagship Pioneering, from time to time as needed. The Company reported general and administrative expense totaling $0 million and $0.2 million for the years ended December 31, 2019 and 2018, respectively.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”).
Principles of Consolidation	Principles of Consolidation The accompanying consolidated financial statements reflect the operations of the Company and its wholly owned subsidiaries. All intercompany accounts and transactions have been eliminated.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Actual results may differ from those estimates.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash includes cash in readily available checking accounts. The Company’s cash deposits on hand at one financial institution often exceed federally insured limits. Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase.
Restricted Cash	Restricted Cash Restricted cash is cash that is restricted as to withdrawal or use under the terms of certain contractual agreements. The restricted cash consists of cash collateral for secured letters of credit for the security deposit on the Company’s leased laboratory and office facilities.
Concentrations of Credit Risk and of Significant Suppliers	Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. The Company’s cash equivalents as of December 31, 2019 consisted only of money market funds. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company relies, and expects to continue to rely, on a small number of vendors to manufacture supplies and raw materials for its development programs. These programs could be adversely affected by a significant interruption in these manufacturing services or the availability of raw materials.
Property and Equipment	Property and Equipment Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the respective assets. Laboratory and office equipment, computer equipment and furniture and fixtures are depreciated over a period of five years, and leasehold improvements are amortized over the lesser of the asset’s estimated useful life or the remaining lease term. Major additions and betterments are capitalized; maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operating expenses as incurred.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset may not be recoverable. When such events occur, the Company compares the carrying amounts of the assets to their undiscounted expected future cash flows. If this comparison indicates that there is an impairment, the amount of the impairment is calculated as the difference between the carrying value and the fair value. The Company has not recorded any impairment charges in the periods presented.
Deferred Issuance Costs	Deferred Issuance Costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred issuance costs until such financings are consummated. After consummation of an equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the offering. The Company capitalizes certain legal and other third‑party fees that are directly associated with obtaining access to capital under credit facilities. Deferred financing costs related to a recognized debt liability are recorded as a reduction of the carrying amount of the debt liability and amortized to interest expense using the effective interest method over the repayment term.
Segment Information	Segment Information Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision-making group, in making decisions on how to allocate resources and assess performance. The Company’s chief operating decision maker is the chief executive officer (“CEO”). The Company and CEO view the Company’s operations and manage its business as one operating segment.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and bonuses, stock‑based compensation, employee benefits, facilities costs, laboratory supplies, depreciation, manufacturing expenses and external costs of vendors engaged to conduct preclinical development activities and clinical trials, as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology are expensed as research and development in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed.
Research and Manufacturing Contract Costs and Accruals	Research and Manufacturing Contract Costs and Accruals The Company has entered into various research and development and manufacturing contracts. These agreements are generally cancelable, and related payments are recorded as the corresponding expenses are incurred. The Company records accruals for estimated ongoing costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the research studies or clinical trials and manufacturing activities, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be required in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.
Patent Costs	Patent Costs All patent‑related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Income Taxes	Income Taxes Deferred tax assets and liabilities are recognized for the expected future tax consequences of events that have been included in the consolidated financial statements or tax returns. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax asset to an amount, which, more likely than not, will be realized. The Company recognizes the tax benefit from any uncertain tax positions only if it is more likely than not that the tax position will be sustained on examination by the taxing authorities, based on the technical merits of the position. The tax benefits recognized in the consolidated financial statements from such a position are measured based on the largest benefit that has a greater than fifty percent likelihood of being realized upon ultimate settlement.
Fair Value Measurements	Fair Value Measurements Certain assets and liabilities were carried at fair value. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1 – Unadjusted quoted prices in active markets that are accessible to the reporting entity at the measurement date for identical assets and liabilities. • Level 2 – Inputs other than quoted prices in active markets for identical assets and liabilities that are observable either directly or indirectly for substantially the full term of the asset or liability. Level 2 inputs include the following: • quoted prices for similar assets and liabilities in active markets • quoted prices for identical or similar assets or liabilities in markets that are not active • observable inputs other than quoted prices that are used in the valuation of the asset or liabilities (e.g., interest rate and yield curve quotes at commonly quoted intervals) • inputs that are derived principally from or corroborated by observable market data by correlation or other means • Level 3 – Unobservable inputs for the assets or liability (i.e., supported by little or no market activity). Level 3 inputs include management’s own assumptions about the assumptions that market participants would use in pricing the asset or liability (including assumptions about risk). The carrying amount of the Company’s other financial assets and liabilities including cash, accounts payable and long-term debt approximate fair value because of the relatively short period of time between origination and expected realization or settlement.
Net Loss Per Share	Net Loss Per Share Prior to the closing of its IPO, the Company followed the two-class method when computing net income (loss) per share as the Company had issued preferred stock shares that met the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon the respective rights to receive dividends as if all income for the period had been distributed. No losses were allocated to the preferred stock. Subsequent to the closing of its IPO, b asic net income (loss) per share is computed by dividing the net income (loss) by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) is computed by adjusting net income (loss) to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share is computed by dividing the diluted net income (loss) by the weighted average number of shares of common stock outstanding for the period, including potential dilutive common shares assuming the dilutive effect of common stock equivalents. The following table presents securities that have been excluded from the computations of diluted weighted-average shares outstanding as they would be anti-dilutive due to the net losses: As of December 31, 2019 2018 Options to purchase common stock 7,285,581 6,686,267 Unvested restricted common stock 1,250 328,624 Convertible redeemable preferred stock (as converted to common stock) — 18,517,386 Warrant to purchase redeemable convertible preferred stock (as converted to common stock) — 68,514 7,286,831 25,600,791
Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock The Company recorded redeemable convertible preferred stock at fair value upon issuance, net of any issuance costs. The Company’s redeemable convertible preferred stock was subject to a dividend when and if declared by the Company’s board of directors (the “Board”). Since inception, no dividend has been declared. The Company classifies stock that is redeemable in circumstances outside of the Company’s control outside of permanent equity. No accretion was recognized as the contingent events that could have given rise to redemption were not deemed probable. Upon completion of the IPO, all the Redeemable Convertible Preferred Stock was converted to shares of common stock.
Stock-Based Compensation	Stock-Based Compensation For stock-based awards, the Company measures the estimated fair value of the stock-based award on the date of grant and recognizes compensation expense for those awards over the requisite service period, which is generally the vesting period of the respective award. For stock-based awards with service-based vesting conditions, the Company records the expense for these awards using the straight-line method. For stock options with performance-based vesting conditions, the Company records the expense for these awards over the requisite service period using an accelerated attribution method to the extent the achievement of the performance condition is probable. The Company accounts for forfeitures as they occur. The Company classifies stock-based compensation expense in its consolidated statements of operations in the same manner in which the award recipient’s cash compensation costs are classified.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. The Company’s comprehensive net loss equals the reported net loss for all periods presented.
Subsequent Events	Subsequent events The Company evaluates events and/or transactions occurring after the balance sheet date and before the issue date of the financial statements to determine if any of those events and/or transactions require adjustment to or disclosure in the financial statements.
Accounting Pronouncements Issued and Not Adopted	Accounting Pronouncements Issued and Not Adopted In February 2016, the FASB issued ASU No. 2016-02, Leases

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Schedule of Securities Excluded from Computation of Diluted Weighted-Average Shares Outstanding	The following table presents securities that have been excluded from the computations of diluted weighted-average shares outstanding as they would be anti-dilutive due to the net losses: As of December 31, 2019 2018 Options to purchase common stock 7,285,581 6,686,267 Unvested restricted common stock 1,250 328,624 Convertible redeemable preferred stock (as converted to common stock) — 18,517,386 Warrant to purchase redeemable convertible preferred stock (as converted to common stock) — 68,514 7,286,831 25,600,791

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Schedule of Assets and Liabilities Carried at Fair Value on a Recurring Basis	The following tables set forth by level, within the fair value hierarchy, the assets and liabilities carried at fair value on a recurring basis (in thousands): Fair Value Measurement as of December 31, 2019 Level 1 Level 2 Level 3 Total Assets: Money market funds included within cash and cash equivalents $ 25,304 — — $ 25,304 Total $ 25,304 — — $ 25,304 Fair Value Measurement as of December 31, 2018 Level 1 Level 2 Level 3 Total Assets: Money market funds included within cash and cash equivalents $ 74,145 — — $ 74,145 Total $ 74,145 — — $ 74,145 Liabilities: Warrant liability $ — — 1,213 $ 1,213 Derivative liability — — 210 $ 210 $ — — 1,423 $ 1,423
Roll-Forward of Aggregate Fair Values of Liabilities Using Level 3 Inputs	The following table presents a roll-forward of the aggregate fair values of the Company’s liabilities for which fair value is determined by Level 3 inputs (in thousands): Warrant Liability Derivative Liability Balance – January 1, 2018 $ 295 $ — Initial fair value of derivative liability — 15 Change in fair value 918 195 Balance – December 31, 2018 1,213 210 Change in fair value through the exercise /settlement date (342 ) 90 Reclassification to additional paid-in capital in connection with IPO (871 ) — Settlement of liability in connection with IPO — (300 ) Balance – December 31, 2019 $ — $ —

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consist of the following (in thousands): As of December 31, 2019 2018 Laboratory equipment $ 4,526 $ 3,226 Office and computer equipment 1,418 1,337 Leasehold improvements 687 653 Construction in process 2,650 698 Property and equipment – at cost 9,281 5,914 Less accumulated depreciation and amortization (2,539 ) (1,221 ) Property and equipment – net $ 6,742 $ 4,693

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2019
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): As of December 31, 2019 2018 Payroll and benefits $ 2,426 $ 3,297 Consulting service 230 243 Legal service 171 90 Research and development 4,259 3,718 Capital lease payable – short term 68 91 Other 1,207 420 $ 8,361 $ 7,859

Debt Financing (Tables)

Debt Financing (Tables)	12 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Summary of Future Principal Payments	Future principal payments under the Credit Agreement as of December 31, 2019 are as follows (in thousands): 2021 5,454 2022 8,182 2023 8,182 2024 682 Total future principal payments 22,500 Less unamortized debt discount 2,109 Total balance $ 20,391

Commitments and contingencies (

Commitments and contingencies (Tables)	12 Months Ended
	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Lease Payments	Future minimum lease payments under the non-cancelable operating leases consisted of the following as of December 31, 2019 (in thousands): Year Ending December 31, 2020 5,843 2021 6,026 2022 6,207 2023 6,393 2024 6,584 Thereafter 32,284 $ 63,337

Stockholders_ Equity (Tables)

Stockholders’ Equity (Tables)	12 Months Ended
	Dec. 31, 2019
Stockholders Equity Note [Abstract]
Summary of Convertible Preferred Stock	As of December 31, 2018, convertible preferred stock consisted of the following: As of December 31, 2018 Preferred Stock Authorized Preferred Stock Issued and Outstanding Carrying Value Liquidation Preference Common Shares Issuable Upon Conversion Series A Preferred Stock 14,469,180 14,383,563 $ 10,487 $ 10,500 7,191,781 Series A-1 Preferred Stock 3,057,972 3,057,972 5,168 5,168 1,528,985 Series B Preferred Stock 9,537,276 9,485,863 36,839 36,900 4,742,924 Series C Preferred Stock 10,107,404 10,107,404 100,732 100,973 5,053,696 37,171,832 37,034,802 $ 153,226 $ 153,541 18,517,386
Assumptions to Determine Grant-Date Fair Value of Options	The assumptions that the Company used to determine the grant-date fair value of options granted were as follows: Years Ended December 31, 2019 2018 Expected volatility 66.1% - 84% 46% - 55% Risk-free interest rate 1.42% - 2.54% 2.66% - 3.07% Expected term (in years) 5.50-6.25 5.81-6.25 Expected dividend yield — % — %
Stock Option Activity	A summary of the Company’s stock option activity and related information is as follows: Options Weighted Average Exercise Price Weighted Average Remaining Life (in Years) Aggregate Intrinsic Value Outstanding as of January 1, 2019 6,686,267 $ 7.50 9.2 68,167 Granted 2,543,902 8.7 Exercised (445,160 ) 1.41 Canceled (1,499,428 ) 8.15 Outstanding as of December 31, 2019 7,285,581 $ 8.15 8.7 5,075 Options exercisable as of December 31, 2019 1,900,395 7.3 8.1 2,158 Options vested or expected to vest as of December 31, 2019 7,285,231 8.2 8.7 5,075
Restricted Common Stock Activity	The following table summarizes the Company’s restricted common stock activity for the year ended December 31, 2019: Number of Restricted Shares Weighted-Average Grant Date Fair Value Issued and unvested as of January 1, 2019 328,624 $ 2.19 Vested (327,374 ) Issued and unvested as of December 31, 2019 1,250 $ 2.22
Allocated Stock-based Compensation Expense	The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations: Year Ended December 31, 2019 2018 Research and development $ 3,245 $ 1,309 General and administrative 6,823 5,655 $ 10,068 $ 6,964

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Schedule of Net Deferred Tax Assets and Liabilities	Significant components of the Company’s net deferred tax assets at December 31, 2019 and 2018 are as follows: Years ended December 31, 2019 2018 Deferred tax assets Stock-based compensation $ 3,754 $ 1,067 Net operating loss carryforwards 45,374 24,638 Credit carryforwards 6,519 4,252 Intangible assets 180 197 Charitable contributions 1 1 Accrued expenses 1,125 788 Total deferred tax assets 56,953 30,943 Valuation allowance (56,817 ) (30,837 ) Total net deferred tax assets 136 106 Deferred tax liabilities: Fixed assets (136 ) (106 ) Total net deferred tax liability (136 ) (106 ) Total deferred tax assets (liability) $ — $ —
Schedule of Effective Income Tax Rate Reconciliation	A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Years ended December 31, 2019 2018 Federal income tax expense at statutory rate 21.0 % 21.0 % Stock compensation expense (0.5 ) (1.0 ) Fair value change in warrant liability 0.1 (0.4 ) Permanent differences (0.1 ) (0.1 ) Federal research and development credit 1.7 3.0 State research and development credit 0.9 1.1 State income tax, net of federal benefit 6.3 5.8 Other 0.7 (0.3 ) Change in valuation allowance (30.1 ) (29.1 ) Effective income tax rate 0 % 0 %

Nature of the Business, Basis_2

Nature of the Business, Basis of Presentation, and Going Concern (Details) - USD ($) $ in Thousands	Mar. 04, 2019	Dec. 31, 2019	Dec. 31, 2018
Conversion of Stock [Line Items]
Proceeds from initial public offering		$ 67,761	$ 0
Conversion of stock, shares converted (in shares)			18,517,386
Net loss		86,331	$ 61,744
Net cash used in operating activities		75,796	46,316
Cash and cash equivalents		71,241	76,086
Accumulated deficit		$ (192,559)	$ (106,228)
IPO
Conversion of Stock [Line Items]
Issuance of common stock (in shares)	5,000,000
Proceeds from initial public offering	$ 69,800
Offering costs payable	$ 3,800
Common Stock
Conversion of Stock [Line Items]
Conversion of stock, shares converted (in shares)	18,517,386	18,517,386

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2019USD ($)
Summary Of Significant Accounting Policies [Line Items]
Dividends	$ 0
Laboratory and office equipment
Summary Of Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life	5 years
Computer equipment
Summary Of Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life	5 years
Furniture and fixtures
Summary Of Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life	5 years

Summary of Significant Accoun_5

Summary of Significant Accounting Policies (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities	7,286,831	25,600,791
Redeemable Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities	0	18,517,386
Options to purchase common stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities	7,285,581	6,686,267
Unvested restricted common stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities	1,250	328,624
Warrant to purchase redeemable convertible preferred stock (as converted to common stock)
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities	0	68,514

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Assets and Liabilities Carried at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets, Fair Value Disclosure [Abstract]
Assets, fair value disclosure	$ 25,304	$ 74,145
Liabilities, Fair Value Disclosure [Abstract]
Warrant liability		1,213
Derivative liability		210
Liabilities, fair value disclosure		1,423
Level 1
Assets, Fair Value Disclosure [Abstract]
Assets, fair value disclosure	25,304	74,145
Liabilities, Fair Value Disclosure [Abstract]
Warrant liability		0
Derivative liability		0
Liabilities, fair value disclosure		0
Level 2
Assets, Fair Value Disclosure [Abstract]
Assets, fair value disclosure	0	0
Liabilities, Fair Value Disclosure [Abstract]
Warrant liability		0
Derivative liability		0
Liabilities, fair value disclosure		0
Level 3
Assets, Fair Value Disclosure [Abstract]
Assets, fair value disclosure	0	0
Liabilities, Fair Value Disclosure [Abstract]
Warrant liability		1,213
Derivative liability		210
Liabilities, fair value disclosure		1,423
Money market funds included within cash and cash equivalents
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents fair value disclosure	25,304	74,145
Money market funds included within cash and cash equivalents | Level 1
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents fair value disclosure	25,304	74,145
Money market funds included within cash and cash equivalents | Level 2
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents fair value disclosure	0	0
Money market funds included within cash and cash equivalents | Level 3
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents fair value disclosure	$ 0	$ 0

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details)	1 Months Ended	12 Months Ended
	Mar. 31, 2019USD ($)	Dec. 31, 2019USD ($)	Mar. 04, 2019	Dec. 31, 2018USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Derivative liability				$ 210,000
Asset transfers Level 1 to Level 2		$ 0
Asset transfers Level 2 to Level 1		0
Asset transfers into Level 3		0
Asset transfers out of Level 3		0
Liabilities transfers Level 1 to Level 2		0
Liabilities transfers Level 2 to Level 1		0
Liability transfers into Level 3		0
Liability transfers out of Level 3		0
Derivative Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Success fee	$ 300,000	$ 300,000
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Derivative liability				$ 210,000
Valuation Technique, Option Pricing Model | Level 3 | Measurement Input, Dividend Yield
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrant liability, measurement input		0
Valuation Technique, Probability-Weighted Expected Return Method | Level 3 | Measurement Input, Probability Of Liquidity Event
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Derivative liability, measurement input			1	0.70
Valuation Technique, Probability-Weighted Expected Return Method | Level 3 | Measurement Input, Success Fee
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Derivative liability				$ 300,000
Valuation Technique, Probability-Weighted Expected Return Method | Level 3 | Measurement Input, Discount Rate
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Derivative liability, measurement input			0	0

Fair Value Measurements - Roll-

Fair Value Measurements - Roll-Forward of Aggregate Fair Values of Liabilities Using Level 3 Inputs (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Warrant Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value beginning balance		$ 1,213	$ 295
Change in fair value			918
Change in fair value through the exercise/settlement date		(342)
Reclassification to additional paid-in capital in connection with IPO		(871)
Fair value ending balance			1,213
Derivative Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value beginning balance		210
Initial fair value of derivative liability			15
Change in fair value			195
Change in fair value through the exercise/settlement date		90
Settlement of liability in connection with IPO	$ (300)	$ (300)
Fair value ending balance			$ 210

Property and Equipment, net (De

Property and Equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment – at cost	$ 9,281	$ 5,914
Less accumulated depreciation and amortization	(2,539)	(1,221)
Property and equipment – net	6,742	4,693
Depreciation and amortization	1,318	792
Laboratory equipment
Property Plant And Equipment [Line Items]
Property and equipment – at cost	4,526	3,226
Office and computer equipment
Property Plant And Equipment [Line Items]
Property and equipment – at cost	1,418	1,337
Leasehold improvements
Property Plant And Equipment [Line Items]
Property and equipment – at cost	687	653
Construction in process
Property Plant And Equipment [Line Items]
Property and equipment – at cost	$ 2,650	$ 698

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Payroll and benefits	$ 2,426	$ 3,297
Consulting service	230	243
Legal service	171	90
Research and development	4,259	3,718
Capital lease payable – short term	68	91
Other	1,207	420
Total accrued expenses and other current liabilities	$ 8,361	$ 7,859

Debt Financing - Narrative (Det

Debt Financing - Narrative (Details) - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Oct. 31, 2019
Line of Credit Facility [Line Items]
Loss on extinguishment of debt		$ (580,000)	$ 0
2015 Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 15,000,000	15,000,000
Prepayment fees, amount		$ 100,000
2015 Credit Facility | Line of Credit
Line of Credit Facility [Line Items]
Debt instrument, variable rate, floor		5.75%
Loss on extinguishment of debt		$ (200,000)
2015 Credit Facility | Line of Credit | Prime Rate
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate		1.00%
2019 Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	15,000,000	$ 15,000,000		$ 15,000,000
Prepayment fees, amount	$ 300,000
Prepayment fee	2.00%
2019 Credit Facility | Line of Credit
Line of Credit Facility [Line Items]
Loss on extinguishment of debt	$ (400,000)
Debt Issuance Costs, Net				$ 100,000
2019 Credit Agreement
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	22,500,000	$ 22,500,000
Option to draw down an additional amount	12,500,000
Line of credit facility fees amount payable	$ 300,000
Credit agreement end of term charge percentage	7.55%
debt discount	$ 1,700,000
Credit agreement, expiration period	48 months
Credit agreement interest payment period	15 months
Credit agreement interest payment extension period	24 months
2019 Credit Agreement | Line of Credit
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	4.75%
Debt instrument effective percentage interest rate	8.95%	8.95%
Debt instrument, description of variable rate basis		The principal bears an interest rate of equal to the greater of (i) 8.95% plus the prime rate minus 4.75% and (ii) 8.95%.

Debt Financing - Summary of Fut

Debt Financing - Summary of Future Principal Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Long term debt, net of unamortized debt discount	$ 20,391	$ 14,831
2019 Credit Agreement
Line of Credit Facility [Line Items]
2021	5,454
2022	8,182
2023	8,182
2024	682
Total future principal payments	22,500
Less unamortized debt discount	2,109
Long term debt, net of unamortized debt discount	$ 20,391

Commitments and contingencies -

Commitments and contingencies - Facilities Leases (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Operating lease, term of contract	10 years			10 years
Lease expiration	2029
Rent expense		$ 4.5	$ 1.8

Commitments and contingencies_2

Commitments and contingencies - Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2019USD ($)
Commitments And Contingencies Disclosure [Abstract]
2020	$ 5,843
2021	6,026
2022	6,207
2023	6,393
2024	6,584
Thereafter	32,284
Total future minimum lease payment due	$ 63,337

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Convertible Preferred Stock (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019
Temporary Equity [Line Items]
Preferred Stock Authorized	37,171,832
Preferred Stock Issued	37,034,802
Preferred Stock Outstanding	37,034,802
Carrying Value	$ 153,226	$ 0
Liquidation Preference	$ 153,541
Common Shares Issuable Upon Conversion	18,517,386
Series A Preferred Stock
Temporary Equity [Line Items]
Preferred Stock Authorized	14,469,180
Preferred Stock Issued	14,383,563
Preferred Stock Outstanding	14,383,563
Carrying Value	$ 10,487
Liquidation Preference	$ 10,500
Common Shares Issuable Upon Conversion	7,191,781
Series A-1 Preferred Stock
Temporary Equity [Line Items]
Preferred Stock Authorized	3,057,972
Preferred Stock Issued	3,057,972
Preferred Stock Outstanding	3,057,972
Carrying Value	$ 5,168
Liquidation Preference	$ 5,168
Common Shares Issuable Upon Conversion	1,528,985
Series B Preferred Stock
Temporary Equity [Line Items]
Preferred Stock Authorized	9,537,276
Preferred Stock Issued	9,485,863
Preferred Stock Outstanding	9,485,863
Carrying Value	$ 36,839
Liquidation Preference	$ 36,900
Common Shares Issuable Upon Conversion	4,742,924
Series C Preferred Stock
Temporary Equity [Line Items]
Preferred Stock Authorized	10,107,404
Preferred Stock Issued	10,107,404
Preferred Stock Outstanding	10,107,404
Carrying Value	$ 100,732
Liquidation Preference	$ 100,973
Common Shares Issuable Upon Conversion	5,053,696

Stockholders' Equity - Narrativ

Stockholders' Equity - Narrative (Details)	12 Months Ended
	Dec. 31, 2018shares
Equity [Abstract]
Conversion of stock, shares converted (in shares)	18,517,386

Stockholders' Equity - Common s

Stockholders' Equity - Common stock (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Class Of Stock Disclosures [Abstract]
Number of votes per share of common stock	one
Shares authorized (in shares)		160,000,000
Common shares authorized (in shares)	150,000,000	150,000,000	66,000,000
Common shares par value (in dollars per share)	$ 0.001	$ 0.001	$ 0.001
Preferred shares authorized (in shares)	10,000,000	10,000,000	0
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001	$ 0.001

Stockholders' Equity - Stock-ba

Stockholders' Equity - Stock-based compensation (Details) - shares	Feb. 27, 2019	Dec. 31, 2019
2019 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued (in shares)	2,168,976
Percent of outstanding shares	4.00%
2019 Employee Stock Purchase Plan | Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of outstanding shares	1.00%
Number of shares reserved for issuance (in shares)	180,748
Number of additional shares allowable under the plan (in shares)	542,244
Shares issued during period (in shares)		0

Stockholders' Equity - Assumpti

Stockholders' Equity - Assumptions to Determine Grant-date Fair Value of Options (Details) - Stock Option	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, minimum	66.10%	46.00%
Expected volatility, maximum	84.00%	55.00%
Risk-free interest rate, minimum	1.42%	2.66%
Risk-free interest rate, maximum	2.54%	3.07%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5050 years	5 years 9 months 21 days
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 3 months	6 years 3 months

Stockholders' Equity - Stock Op

Stockholders' Equity - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Options
Outstanding at beginning of period (in shares)	6,686,267
Granted (in shares)	2,543,902
Exercised (in shares)	(445,160)
Canceled (in shares)	(1,499,428)
Outstanding at end of period (in shares)	7,285,581	6,686,267
Options exercisable (in shares)	1,900,395
Options vested or expected to vest (in shares)	7,285,231
Weighted Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 7.50
Granted (in dollars per share)	8.7
Exercised (in dollars per share)	1.41
Canceled (in dollars per share)	8.15
Outstanding at end of period (in dollars per share)	8.15	$ 7.50
Options exercisable (in dollars per share)	7.3
Options vested or expected to vest (in dollars per share)	$ 8.2
Weighted Average Remaining Life and Aggregate Intrinsic Value
Options outstanding, weighted average remaining life (in years)	8 years 8 months 12 days	9 years 2 months 12 days
Options exercisable, weighted average remaining life (in years)	8 years 1 month 6 days
Options vested or expected to vest, weighted average remaining life (in years)	8 years 8 months 12 days
Options outstanding, aggregate intrinsic value	$ 68,167
Fair value of options granted during period (in dollars per share)	$ 5.96	$ 5.47
Unrecognized compensation expense	$ 28,200
Stock Option
Weighted Average Remaining Life and Aggregate Intrinsic Value
Unrecognized compensation expense, weighted average remaining term	2 years 9 months 25 days

Stockholders' Equity - Restrict

Stockholders' Equity - Restricted Common Stock Activity (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)Employee$ / sharesshares	Dec. 31, 2018USD ($)$ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of employees that signed agreements for early exercise of stock options | Employee	7
Early exercise of employee stock options (in shares)	1,295,699
Restricted shares repurchase liability | $	$ 3	$ 720
Restricted Stock
Number of Restricted Shares
Issued and unvested beginning of period (in shares)	328,624
Vested (in shares)	(327,374)
Issued and unvested end of period (in shares)	1,250
Weighted-Average Grant Date Fair Value
Issued and unvested (in dollars per share) | $ / shares	$ 2.22	$ 2.19

Stockholders' Equity - Stock-_2

Stockholders' Equity - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 10,068	$ 6,964
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	3,245	1,309
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 6,823	$ 5,655

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Line Items]
Provision for income taxes	$ 0
Net operating loss carryforwards	$ 128,000,000
Net operating loss carryforwards, expire date	Dec. 31, 2035
Tax credit carryforwards, expire date	Dec. 31, 2039
Increase in valuation allowance	$ 26,000,000	$ 18,000,000
Unrecognized tax positions	0	0
Accrued interest or penalties	0	$ 0
U.S. Federal
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	166,800,000		$ 38,800,000
State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	163,600,000
Commonwealth of Massachusetts | State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 163,600,000
Net operating loss carryforwards, expire date	Dec. 31, 2035
Research and Development | U.S. Federal
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 4,500,000
Research and Development | State
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 2,500,000

Income Taxes - Schedule of Net

Income Taxes - Schedule of Net Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets
Stock-based compensation	$ 3,754	$ 1,067
Net operating loss carryforwards	45,374	24,638
Credit carryforwards	6,519	4,252
Intangible assets	180	197
Charitable contributions	1	1
Accrued expenses	1,125	788
Total deferred tax assets	56,953	30,943
Valuation allowance	(56,817)	(30,837)
Total net deferred tax assets	136	106
Deferred tax liabilities:
Fixed assets	(136)	(106)
Total net deferred tax liability	$ (136)	$ (106)

Income Taxes - Schedule of Effe

Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal income tax expense at statutory rate	21.00%	21.00%
Stock compensation expense	(0.50%)	(1.00%)
Fair value change in warrant liability	0.10%	(0.40%)
Permanent differences	(0.10%)	(0.10%)
Federal research and development credit	1.70%	3.00%
State research and development credit	0.90%	1.10%
State income tax, net of federal benefit	6.30%	5.80%
Other	0.70%	(0.30%)
Change in valuation allowance	(30.10%)	(29.10%)
Effective income tax rate	0.00%	0.00%

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Investor | Flagship Pioneering
Related Party Transaction [Line Items]
General and administrative expense	$ 0	$ 0.2